The risk associated with smoking three or four cigarettes a day is less than for someone who smokes forty cigarettes a day.
But for a particular person, it can be very hazardous indeed, a bigger environmental hazard than almost anything you can name.
Even one cigarette a day raises the risk of cardiovascular disease and cancer.
One cigarette has about fourteen thousand micrograms of tar, but the Environmental Protection Agency standard for inhalable particles is only fifty micrograms per cubic meter of air.
A typical person might breathe about twenty cubic meters of air a day, so with one cigarette, the risk just from particles is about fourteen times as bad as the risk from breathing marginally polluted air.
If you add a factor to take account of the chemical hazards, it is much worse.
In tobacco smoke in general there are sixty known or suspected carcinogens.